The invention is directed to a process for the chemical-thermal decomposition of highly halogenated hydrocarbons by reaction with calcium oxide and/or calcium hydroxide in an above stoichiometric ratio at a temperature of 600.degree. to 800.degree. C. in a reactor.
Highly halogenated hydrocarbons are frequently employed in industry and research. Thus, fluorohydrocarbons serves as propellant gases and refrigerants and are the starting materials for the production of chemically very resistant synthetic resins. Chlorohydrocarbons are employed in large amounts as defatting agents in metal working operations. Further areas of use are chemical purifications of all types. Furthermore, the chlorohydrocarbons are starting materials for the production of polymers, pesticides, and herbicides. Especially because of their high chemical and thermal resistance, the polychlorinated hydrocarbons are employed as heat transfer oils or hydraulic fluids. The polychlorinated biphenyls (PCB) are typical members of this class of material.
Although insofar as this is industrially possible and economically suitable, use is made of the possibility of recycling used halohydrocarbons, there accumulate in the Federal Republic of Germany yearly about 30,000 to 40,000 metric tons of chlorohydrocarbons having a chlorine content of &gt;20% which must be disposed of.
With these so-called special wastes in addition to residues from recycling plants and production residues, it is a question of materials whose use from the point of view of safety and the industrial environment is continuously being reduced and which finally must be disposed of. The best known example of these materials are PCB which in the past were employed chiefly as transformer oils and as dielectrics in condensers. Through the exchange of these liquids for replacement materials alone, it has been calculated that for the Federal Republic of Germany in the next ten years there will be the need to dispose of about 6,000 metric tons yearly of polychlorinated biphenyls.
At the present time, the only prominent possibility for disposal of halohydrocarbons is burning at sea. International agreements (Oslo and London conventions), however, have the goal is to completely limit the burning at sea until the end of this decade. As the alternative thereto, there only remains burning on land. The burning of halohydrocarbons, especially fluorinated and highly chlorinated hydrocarbons in existing special waste combustion plants is problematical. The basic reasons for the difficulties are the danger of corrosion to the brickwork and the waste gas zone because of high crude gas loading of a hydrogen halide (HF and HCl), the emission situation, especially in burning fluorinated hydrocarbons and the high energy input.
Especially through the circumstance that with insufficient burning conditions with the burning of chlorohydrocarbons, highly toxic polychlorinated dibenzodioxine and dibenzofurane can be formed, there is placed increasing criticism on this disposal practice.
In German OS No. 3028193 (and related Hofmann U.S. application Ser. No. 287,120 filed June 27, 1981, the entire disclosure of which is hereby incorporated by reference and relied upon), there is described a process for the pyrolytic decomposition of halogen and/or phosphorus containing organic materials in which these are reacted with an above stoichiometric ratio of calcium oxide or calcium hydroxide at a temperature of 300.degree.0 to 800.degree. C. in a reactor.
The disadvantage with this process is that not all halohydrocarbons can be destroyed without problems. The temperatures necessary for the quantitative decomposition of the chemically and thermally very stable highly halogenated hydrocarbons among which there must be especially considered the polychlorinated biphenyls is above 600.degree. C. Above this temperature, mixtures of CaO and Ca(OH).sup.2 with the corresponding calcium chlorides form melts. This fact makes considerable difficulties since the necessary continuous throughput of solids through the reactor is hindered thereby and under some conditions is even impossible. In addition to the industrial process difficulties, the formation of melts simultaneously leads to an increased lowering of the rate of decomposition of the halogenated hydrocarbons. This is traced to the severe reduction of the surface area of the solid reactants, which exerts a considerable influence on the reaction in gas-solid reactions. Even a large excess of the basic compounds mentioned is not able to prevent a formation of melts at temperatures above 600.degree. C. with subsequent incrustation in the cooling-off phase.
Therefore, it was the problem of the present invention to develop a process for the chemical-thermal decomposition of highly halogenated hydrocarbons by reaction with an above stoichiometric ratio of calcium oxide and/or calcium hydroxide at a temperature of 600.degree. to 800.degree. C. in a reactor in which no melts form and in which the waste gases are free of halogen and especially are free of dioxin.